[clang] const: Assertion `D->isCompleteDefinition() && "Cannot layout type before complete!"' failed in clang/lib/AST/RecordLayoutBuilder.cpp when using "-std=c++23"

We recently discovered our code base will failed to build when enable assertion on ToT clang. We created a simplified reproducer:

Code:

```c++
template
struct PointerToMemberTraits;

template
struct PointerToMemberTraits {
using class_type = Class;
using member_type = T;
};

template ::class_type>
constexpr auto& MemberOf(Class& ptr) {
return ptr.*kMemberPtr;
}

template
constexpr auto ContainerOf(Member* ptr) {
return (typename PointerToMemberTraits::class_type*)nullptr;
}

template
struct CustomFutureList {
using value_type = decltype(*kGetFutureImpl((int*)nullptr));
};

template
using FutureList = CustomFutureList, MemberOf>;

struct BaseChannelFuture {
int core_;
using List = FutureList<&BaseChannelFuture::core_>;
};
```

Compiler options: "-std=c++23"

Failure we got:

Assertion:

```console
llvm: clang/lib/AST/RecordLayoutBuilder.cpp:3390:
const ASTRecordLayout &clang::ASTContext::getASTRecordLayout(const RecordDecl *) const:
Assertion `D->isCompleteDefinition() && "Cannot layout type before complete!"' failed.
```
backtrace:

```console
PLEASE submit a bug report to [https://github.com/llvm/llvm-project/issues/](https://www.google.com/url?sa=D&q=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fissues%2F) and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0. Program arguments: /mnt/nvme_crypt/SRC/clang/clang_tot_ci/bin/llvm clang++ -cc1 -std=c++23 channel-21896f.cpp
1. channel-21896f.cpp:31:18: at annotation token
2. channel-21896f.cpp:29:1: parsing struct/union/class body 'BaseChannelFuture'
3. channel-21896f.cpp:12:17: instantiating function definition 'MemberOf<&BaseChannelFuture::core_, BaseChannelFuture>'
#0 0x00005635b1cfacf8 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/mnt/nvme_crypt/SRC/clang/clang_tot_ci/bin/llvm+0x9ac6cf8)
./channel-21896f.sh: line 4: 2943618 Aborted (core dumped) "/mnt/nvme_crypt/SRC/clang/clang_tot_ci/bin/llvm" "clang++" "-cc1" "-std=c++23" "channel-21896f.cpp"
```

This was tested on clang revision 9de31c4a3e77ceb1e736d4df3ae9e3f5a1dc8748 and e0f5ad74be2b2f127442f721c70c6753c41e6d8c. Both will fail due to this assertion error.

If we delete the "-std=c++23" from the reproducer, the same code pass the build without issue.

Packed reproducer: [repro_shrink.tar.gz](https://github.com/user-attachments/files/26767915/repro_shrink.tar.gz)

Start with the reproducer and run it using clang++ -cc1 -std=c++23 to confirm the assertion in clang/lib/AST/RecordLayoutBuilder.cpp. Trace the instantiation shown in the stack dump, beginning at MemberOf and the BaseChannelFuture definition. Done means the reproducer builds without the assertion while preserving the existing non-C++23 behavior.

Written by the indexing model from the issue text.